1. Field of the Invention
This invention relates generally to intercommunication systems and more particulatly, but not by way of limitation, to intercommunication systems having apparatus whereby two-way communication can be maintained between the calling station and the called station or stations.
2. Description of the Prior Art (Prior Art Statement)
The following statement is intended to be a prior art statement in compliance with the guidance and requirements of 37 C.F.R. 1.56, 1.97 and 1.98.
Several intercom systems are known to be, or to have been, used. These systems are believed to be classifiable in one of two categories. The first category includes those systems having customized telephones which include special built-in electronic circuitry. The second category includes those types of systems which have large boxes or housings used in addition to the telephones with which such systems are used.
Regardless of the classification, however, these presently known types generally use unbalanced lines to interconnect the individual local stations with a central control unit. Also, each of these types uses various types of electronic switching circuits to perform the necessary functions of interconnecting the calling and called stations.
These systems generally indicate the need for some type of intercommunication system. More particularly, our present manner of living creates the need for intercommunication systems in such places as the home, business and hospital so that rapid communication may be spread to diverse and distant locations.
To satisfy such needs in an economically feasible manner, there is the further need to utilize ordinary telephones which are normally present in such places as the home, business and hospital. Further, it is important that the intercom system be simple to install so that installation and service costs may be kept to a minimum.
Although these needs are important, it is also important to provide an intercom system which provides relatively high quality audio so that the communications can be understood. Furthermore, there is the need that the system be versatile so that either individual or multiple stations may be addressed and also so that the communication can be broadcast in either a conversational level or a power amplification level. In such a versatile system wherein either an individual station or a plurality of stations may be called, it is important to provide the conversational level with separate output level controls so that one lever can be used when the communication is between only the calling and a single called station, for example, and a separate lever can be used when all of the stations are being addressed.
Still another need is to provide an intercommunication system which permits two-way conversation. To provide such a two-way system creates the further need for the system to have a talk channel and a listen channel which are interconnected to prevent lock up in either the listen or the talk modes. Furthermore, such interconnected channels raise the need for a built-in balance detector to test and set the balance for the least power feedback between the talk and listen channels. Another need in such a two-way system is the provision of apparatus for generating warning, or notification, signals to advise local stations that have been called that they are actively connected to a calling station.
In addition to these many needs, there is the need for apparatus within the intercommunication system whereby a central operator may directly select and communicate with any one or more of the various local stations.
Although as previously stated, there are various types of intercommunication systems presently known, it is believed that each of these systems has the shortcoming of failing to meet each and every one of these needs.